A fastener based on a permanent magnet is available in variety of structures depending upon the purpose for which it is designed. Such a fastener has been widely used for a variety of purposes, for its simpleness and ease of use and handling. One typical fastener based on permanent magnet is described in Japanese utility model JP 54-3923. This fastener has a female fastening element and a male fastening element. The female element has a permanent magnet with a hole and a ferromagnetic plate with a rod extending from one side of it. The plate is attached to the magnet at one pole side with the rod extending into the hole. The male fastening element has a ferromagnetic plate with a projection extending from one side of it. The magnet hole is adapted to accommodate insertion of the projection from the opposite side or opposite pole of the magnetic. This design makes it possible for the male or female element to come together and be separated. The structure disclosed in Japanese utility model JP 54-3923 is such that the magnet serves as a component of both the female and male fastening elements. The magnetism is furthermore enhanced by the plates and the projection, both of which serve to complete the magnetic circuit. Thus, a fastener disclosed in the Japanese utility model JP 54-3923 provides strong magnetic attraction.
Although this type of fastener provides a good holding strength, however, it tends to be heavy because it uses solid metal plates with solid projections or rods. To this end, the fastener under described in U.S. Patent Des. 247,468 contemplates use of a hollow rod, thus reducing the fastener weight. Although the weigh reduction is desirable, the hollow rod concept has not been utilized and no consideration was given to what magnetic force is best suited for a given fastener. In fact, no attempt has even been made in the past to determine an optimum relationship between the extent of weight reduction and magnetic force required, when a hollow rod, as opposed to a solid rod, is used.